The following timetable is provided by the National September 11th Memorial and Museum to help people fully understand the events that occurred on that late summer morning 20 years ago.

On the morning of September 11, 2001, 19 terrorists hijacked four commercial aircraft flying to California. Planes take off from Boston Airport; Newark, New Jersey; Washington, D.C. September 11 will be a notorious day in American and world history, and the events that happened that day will change the world forever. As the world commemorates the 20th anniversary of 9/11, the following timetable (provided by the National September 11 Memorial Museum) can help people fully understand the events that took place on that late summer morning 20 years ago.

5:45 am: Two hijackers pass through security at Portland International Airport in Maine. These people will take a short flight to Boston Logan International Airport, where they will board American Airlines Flight 11 with three other hijackers. ¥6:00: Two of the hijacked planes, American Airlines Flight 11 and United Airlines Flight 175, will eventually crash into the North and South Towers of the World Trade Center complex in New York City. This day is an important day on the New York City political calendar, as the polling station opens at 6 a.m. for primary elections.

7:59 in the morning: American Airlines Flight 11 departed from Boston with 11 crew members, 76 passengers and 5 hijackers on board. The plane will eventually crash into the North Tower of the World Trade Center, which is filled with more than 76,000 pounds of fuel.

8:15 am: United Airlines Flight 175 departs from Boston with 9 crew members, 51 passengers and 5 hijackers on board. The flight was also loaded with 76,000 pounds of fuel.

8:19 am: Flight attendant Betty Ann Ong reminds American Airlines ground crew that Flight 11 has been hijacked. The call lasted approximately 25 minutes, and Ong reported that it could not reach the cockpit. Just before Ong called, one of the hijackers stabbed Daniel M. Lewin who was sitting in front of him in the first class cabin. Lewin is likely to be the first person killed in the 9/11 attacks.

8:20 am: American Airlines Flight 77 departs from Washington Dulles International Airport. The flight has 49,900 pounds of fuel, 6 crew members, 53 passengers and 5 hijackers.

8:21 am: The transponder on flight 11 shuts down. The device is designed to allow air traffic controllers to identify and monitor the flight path of the aircraft.

8:24 am: A hijacker on Flight 11 unknowingly broadcast a message to the air traffic controller to alert them to the attack. The hijacker tried to communicate with the passengers and crew in the cabin.

8:30 am: Around this time, about 80 people have begun to gather on the 106th floor of the North Tower of the World Trade Center to participate in the financial technology conference. The meeting was one of the many events on the schedule of the Trade Center that day.

8:37 am: The Boston Air Traffic Control Center issues a hijacking alert to the military.

8:42 am: United Airlines Flight 93 departs from Newark International Airport. The flight was originally scheduled to take off at roughly the same time as the other hijacked aircraft, but it was delayed due to daily traffic. There were 7 crew members, 33 passengers and 4 hijackers on board. The flight was filled with 48,700 pounds of fuel.

8:46 am: Five hijackers crashed Flight 11 into the 93-99 floors of the North Tower of the World Trade Center. Hundreds of people, including everyone on board, died immediately. The crash cut off all three emergency stairwells and trapped hundreds of people above the 91st floor.

8:46 am: Police, paramedics and firefighters are dispatched to the North Tower.

8:50 am: While visiting an elementary school in Florida, US President George W. Bush was notified that a small plane had crashed into the North Tower.

8:52 am: A flight attendant on Flight 175 arrives at a United Airlines in San Francisco and reports that the flight has been hijacked. By 9 a.m., multiple passengers on Flight 175 had called their family members.

8:55 am: The Port Authority informs people in the South Tower through the public address system that the building is safe and does not need to be evacuated.

8:59 am: The Port Authority Police Department ordered the evacuation of the two towers. A minute later, Captain Anthony Whitaker expanded the order to include all civilians in the entire World Trade Center complex.

9:02 am: The evacuation order is broadcast on the South Tower.

9.03 am: Five hijackers crashed Flight 175 into the 77th to 85th floors of the South Tower. Everyone on the plane was killed, and the number of people in the building is unknown. Two of the three emergency stairwells were impassable, most elevator cables were cut, and many people were trapped above the impact zone and in the elevator car.

9:03 am: The second call to mobilize brings the total number of New York City Police Department officers who responded to the scene to approximately 2,000. In addition, FDNY issued the fifth alert and deployed hundreds of additional firefighters to the scene.

9:05 am: President Bush is informed that the second plane has crashed in the World Trade Center.

9:12 am: Flight attendant Rene A. May called her mother and told her that the hijacker had taken control of Flight 77. When May's phone disconnected, she called American Airlines.

9:30 am: Amid reports of more aircraft being hijacked, the Mayor’s Emergency Management Office at 7 World Trade Center was evacuated.

9:37 am: The hijacker crashed Flight 77 into the Pentagon. In the fire caused by the crash, all 53 passengers and 6 crew members were killed, and 125 military and civilian personnel on the ground were killed.

9:42 am: The US Federal Aviation Administration grounds all flights and orders all civilian aircraft in US airspace to land. It is also prohibited to leave the country.

9:45 am: The White House and the U.S. Capitol begin to evacuate. When the evacuation began, both the House of Representatives and the Senate were in meetings.

9:58 AM: Flight 93 was so low that passenger Edward P. Felt was able to contact the emergency 911 operator in Westmoreland County, Pennsylvania.

9:59 am: The South Tower collapses after burning for 56 minutes. The tower collapsed in just 10 seconds.

9:59 am: The government continuity procedure is implemented for the first time in American history.

10:03 AM: Four hijackers crashed Flight 93 into a field near Shanksville, Pennsylvania. All 33 passengers and 7 crew members on board were killed. Passengers and crew rushed into the cockpit and the plane crashed within only 20 minutes of flight time from Washington, DC

10:15 am: The E-ring of the Pentagon collapses.

10:28 am: The North Tower collapses after 102 minutes of burning. More than 1,600 people were killed in the attack on the North Tower.

11:02 am: New York City Mayor Rudolph Giuliani urges evacuation from Lower Manhattan.

12:16 pm: The last flight is still over the continental United States. In two and a half hours, US airspace has cleared approximately 4,500 commercial and general aviation aircraft.

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